DESCRIPTION: The Calponin-Homology (CH) domain identifies a new super family of cytoskeletal proteins that integrate the cytoskeleton and signalling pathways. Based on a small actin binding domain from calponin, the CH domain functions as a module that targets various proteins including signaling proteins, vav and IQGAP, and actin crosslinking proteins to actin filaments. More recently, CH domains were discovered in the IFAPs plectin and BPAB1n1 (dystonin) suggesting that they connect the actin and intermediate filament cytoskeletons. This widespread use of CH domains in important structural and signaling systems may provide a direct cellular mechanism for regulating cell structure. To understand how CH domain proteins organize the cytoskeleton, this proposal has three specific aims. The first goal is to describe the mechanism of cross linking by fibrin by determining the 3D structure of a fimbrin-actin cross link and by identifying interacting residues at the binding interface by site directed mutagenesis. This structure will serve as a model for understanding how CH domain proteins bind the actin cytoskeleton. The second aims to describe the function of the fibrin-vimentin complex at cell substratum adhesion sites. Interactions between the actin and IF cytoskeleton may play in important role in the assembly and/or stability of a cell attachment. The last aim is to identify the function of calponin by a combination of biochemical and genetic approaches. In the simple cytoskeleton of yeast which lacks the muscle contractile system, calponin should more closely represent its function in non-muscle cells. The applicant points out that studies on CH domain proteins are directly relevant to understanding underlying mechanisms of disease. The oncogenic properties of vav, the onset of myotonic dystony, blood disorders, and muscular dystrophy are caused by defects in different members of the CH domain superfamily.